1. Field of the Invention
The present invention relates to a failure detecting apparatus for an anti-skid brake control system of an automobile or motor vehicle, which apparatus is designed for detecting occurrence of a failure or fault in an anti-skid brake control system (ABS) employed in the motor vehicle for the purpose of controlling the braking efforts applied to the wheels of the motor vehicle on the basis of wheel speed signals. More particularly, the invention is concerned with a failure detecting apparatus for ah anti-skid brake control system of a motor vehicle which apparatus can be implemented inexpensively while ensuring a high reliability by using in association with a main microcomputer an auxiliary microcomputer of lower performance as compared with that of the former.
2. Description of the Related Art
In the automobiles and motor vehicles, there has heretofore been employed a failure detecting apparatus for diagnosing an anti-skid (or anti-lock) brake control system which is designed for controlling the braking efforts applied to individual wheels of the motor vehicle on the basis of the wheel speed signals for optimizing the skid or slip thereof. A typical one of such failure detecting apparatuses is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 267140/1986 (JP-A-61-267140).
For better understanding of the invention, the related background techniques will first be described in some detail. FIG. 11 is a block diagram showing schematically a structure of a failure detecting apparatus for an anti-skid brake control system of a motor vehicle which is described in the publication mentioned above. Referring to the figure, a wheel speed detection means generally denoted by a numeral 1 generates wheel speed signals V representing wheel rotation speeds each in terms of pulse frequency and is comprised of four wheel speed sensors 1a to 1d each implemented in the form of a transducer and provided in association with the individual wheels (not shown), respectively, of a motor vehicle (not shown either). The wheel speed signals V outputted from the wheel speed sensors 1a to 1d are supplied to a signal processing circuit 2 to undergo amplification as well as signal conditioning processing such as waveform shaping.
There are disposed in parallel a pair of microcomputers 3a and 3b, wherein the wheel speed signals V outputted from the signal processing circuit 2 are supplied to the microcomputers 3a and 3b via a multiple-line bus L1. The microcomputers 3a and 3b are constituted by circuit systems implemented in the form of program-controlled circuits or microprocessors of a same structure or configuration for processing the wheel speed signals V with a redundancy in order to ensure a fail-safe operation of the anti-skid brake control system. In other words, the wheel speed signals V are processed simultaneously by both of the microcomputers 3a and 3b. At this juncture, it should however be mentioned that one of the microcomputers 3a and 3b (e.g. the microcomputer 3a) serves as a main microcomputer for controlling the anti-skid brake system while the other (3b) is destined to serve for a failure monitoring function which will be described later on. Signal buses L2 and L3 are led out from output ports of the microcomputers 3a and 3b , while signal buses L4 and L5 are led out from logic blocks (described below) incorporated in the microcomputers 3a and 3b, respectively.
More specifically, the microcomputers 3a and 3b include the logic blocks 6a and 6b which fetch the wheel speed signals V via the signal bus L1, comparators 9a and 9b for comparing the signals on the signal buses L4 and L5 with the signals on the output ports L2 and L3, respectively, and clock oscillators 22 and 23, respectively.
The microcomputers 3a and 3b are connected to each other via synchronizing signal lines 19. Crystal elements 20 and 21 cooperate with the clock oscillators 22 and 23 to constitute synchronizing means for establishing synchronism in operations of both the microcomputers 3a and 3b.
The microcomputer 3a is supplied with an electric power from a power source. Bat via a main switch 12.
An amplifier circuit serving as a valve driver 13 is connected to the output port L2 of the microcomputer 3a for electrically driving a multi-way hydraulic valve 10a which constitutes a plurality of hydraulic actuators for a brake system. The multi-way hydraulic valve 10a has a plurality of outlet ports connected to, for example, brake pipes (not shown) disposed between a master cylinder of the motor vehicle and wheel brakes thereof, although not shown.
Thus, the multi-way hydraulic valve 10a operates in response to a hydraulic pressure control signal supplied from the microcomputer 3a via the signal bus L2 and the valve driver 13 to thereby control the braking efforts to be optimal so that the slip of each wheel can be maintained optimal. The degree of the slip is arithmetically determined by the microcomputers 3a and 3b on the basis of the wheel speed signal V. The control of the slip can be effectuated by increasing, decreasing or holding constant the brake pipe pressure.
A feedback circuit 14 is provided for feeding back the output signal of the microcomputer 3a via the valve driver 13 to the comparator 9b incorporated in the microcomputer 3b. Further provided are count/evaluation circuits 15a and 15b which serve to count the output signal pulses of the comparators 9a and 9b, respectively, for evaluation thereof. Output signals of the count/evaluation circuits 15a and 15b are monitored by monitor circuits 16a and 16b, respectively, which are also in charge of controlling the main switch 12 in accordance with the result of the monitoring.
When the output from either one of the monitor circuits 16a and 16b indicates occurrence of a failure in the microcomputer 3a or 3b, the main switch 12 is instantaneously opened to thereby interrupt the electric power supplied to the microcomputer 3a from the power source Bat, which of course results in that the anti-skid brake control system is set to the state where the anti-skid brake control does not work.
Next, description will now turn to operation of the prior art failure detecting apparatus for the anti-skid brake control system.
The wheel speed signals V outputted from the wheel speed sensor means S1 to S4 provided in association of the wheels of the motor vehicle, respectively, undergo amplification and shaping processing in the signal processing circuit 2 to be thereby converted into binary signals (i.e., data signals) which are then supplied to the microcomputers 3a and 3b via the multiple-line bus L1.
Thus, both of the microcomputers 3a and 3b process in parallel the wheel speed signals V which are in the form of binary signals. The results of the processings are delivered from the output ports L2 and L3, respectively. Thus, it can be said that the processing of the wheel speed signals V is carried out in duplicate or with a redundancy.
The output signal of the microcomputer 3a is transformed into a hydraulic pressure control signal through the valve driver 13 for driving the multi-way hydraulic valve 10a in a manner known per se, whereby the pressures in the brake pipes disposed between the master cylinder and the individual wheel brakes are controlled correspondingly.
In this manner, the multi-way hydraulic valve 10a operates in response to the hydraulic pressure control signal supplied from the microcomputer 3a to thereby optimize the braking efforts so that magnitude of the slip can be set at an optimal value. On the other hand, the slip is detected by the microcomputers 3a and 3b on the basis of the wheel speed signals V. Incidentally, the slip control can be effectuated by increasing, decreasing or holding constant the hydraulic brake pressure.
It is known that the multi-way hydraulic valve 10a is employed in such an arrangement that a pressure increasing valve (booster valve) and a pressure decreasing valve (release valve) are provided for each of the wheels, i.e., for each of the control circuits including the microcomputer 3a.
The operation cycles of the microcomputers 3a and 3b are determined through cooperation of the clock oscillators 22 and 23 and the crystal elements 20 and 21. By way of example, the microcomputers 3a and 3b operate at 10 MHz and are synchronized with each other via the synchronization path 19.
Further, the microcomputers 3a and 3b transfer the wheel speed signals V and the hydraulic pressure control signal for the multi-way hydraulic valve 10a (the pressure increasing signal, pressure hold signal, the pressure decreasing signal and the like) with each other via the signal buses L4 and L5. At the same time, these signals are supplied to the comparators 9a and 9b incorporated in the parallel circuit systems, respectively.
Besides, the hydraulic pressure control signal issued from the microcomputer 3a is transferred to the comparator 9b incorporated in the microcomputer 3b via the valve driver 13 and the feedback circuit 14, while the hydraulic pressure control signal issued from the microcomputer 3b is supplied to the comparator 9a incorporated in the microcomputer 3a. With this arrangement, it is checked whether coincidence exists between the signal generated internally of the microcomputers 3a and 3b and the hydraulic pressure control signals as led outwardly.
The output signals of the comparators 9a and 9b are supplied to the monitor circuits 16a and 16b via the count/evaluation circuits 15a and 15b, respectively. In that case, when either one of the monitor circuits 16a and 16b responds to the input signal (i.e., when occurrence of failure is decided), the main switch 12 is opened to thereby interrupt the power supply to the microcomputer 3a from the power source Bat. Thus, the anti-skid brake control system shown in FIG. 11 is changed over to the state where the system is inoperative.
As will now be understood from the foregoing description, in the conventional anti-skid brake control system such as illustrated in FIG. 11, there are employed two microcomputers 3a and 3b for performing identical arithmetic operations involved in the control, wherein fault or failure is detected on the basis of the result of decision made as to whether the results of operations performed by both the microcomputers 3a and 3b coincide with each other. However, solely for the purpose of detecting occurrence of failure in the anti-skid brake control system (ABS) of the motor vehicle, it seems unnecessary to adopt such configuration that the same processing is performed in duplicate and in parallel, because the anti-skid brake control is simply shut down upon occurrence of failure, differing from a fail-safe redundancy system where the back-up function is required upon occurrence of failure, as can be seen in the case of a brake control system for an air plane.
Thus, the failure detecting apparatus for the anti-skid brake control system (ABS) known heretofore in which two microcomputers of a same performance are employed as a main microcomputer and an auxiliary microcomputer for performing processing and collating operations in parallel through same procedure suffers from a problem that implementation of the apparatus involves unnecessarily high expensiveness.